Tectonic Uplift and Fan Delta Incision along the Conway Coast, NZ

Abstract:

The Conway Coast features a unique confluence of fan deltas, terrace uplift, and transpressional faulting. Thrust faults have caused the uplift of the Hawkeswood Range which provides sediment sources for local Gilbert-type fan deltas uplifting to form terraces. Sediments are characterized by steeply dipping foreset beds, horizontal topset beds and tangential bottomset beds. These fan deltas tend to form on steep basin margins often associated with extensional faulting. However, the Conway coast is dominated by thrust faulting. The Rafa terrace, dated between 52.4±4.5 and 79.0±3.5 ka with Optically Stimulated Luminescence, is exposed in cliff faces on the beach. Local creek channels deposited the fan deltas and later incised older deposits to reveal a three-dimensional perspective of the fan delta. This perspective is vital to the recreation of the fan delta position and channel avulsion, leading to a better understanding of the role of uplift superimposed by sea level change in the formation of the terraces.

Forty-three photographs were stitched together to form an extensive photomosaic. The types of sediments, significant structures and relationships between adjacent beds were noted and analyzed. Correlations between the sedimentary architecture and sea level change were made using a sea level curve of the South Pacific over the last 140 ka. The incision of mud by gravel producing large flame structures suggests that there was a drop in sea level and a subsequent progradation of the fan delta. Based on the sea level curve, this drop occurred between ~66-79 ka. Higher in the fan delta, a fining-upward sequence points to a rise in sea level. This correlates with a rise on the sea level curve between ~52-63 ka. Upsection there is another coarsening-upward sequence of 5-10 m thick beds suggesting a fall in sea level before the uplift and incision of the terrace and the subsequent formation of the Ngaroma terrace. Due to their lateral extent, the gravels seem to result from sea level change, rather than channel avulsion. Although evidence for both sea level change and channel avulsion are present, the evidence for sea level change is more prevalent than for channel avulsion. Thus this study concludes that sea level change played a larger role than the local fan delta in the formation of the terraces along the Conway coast.